Niels Pinkwart Heinz Ulrich Hoppe Lars Bollen Eva Fuhlrott Group-Oriented Modelling Tools with Heterogeneous Semantics. 21-30 2002 conf/its/2002 Intelligent Tutoring Systems http://link.springer.de/link/service/series/0558/bibs/2363/23630021.htm db/conf/its/its2002.html#PinkwartHBF02